In response to the rapid increase in use of information and communications equipment including mobile phones, PHSs (Personal Handyphone System), and PDAs (Personal Digital Assistants), an infrastructure for accessing and transmitting information at any time from any place by anybody has gradually become established. Since these types of information and communications equipment are designed specifically for mobile use, there is demand for light, thin display devices requiring low power input. Liquid crystal display devices (hereinafter LCDs) are thus the major type of display devices currently satisfying such demand. A LCDs information by driving liquid crystal molecules with a few volts of effective voltage to change the light transmissivity. Because liquid crystal itself is a non-light-emitting substance, a separate light source is required, which demands much more power than the power required for driving the liquid crystal. A reflective LCD that utilizes ambient light by providing a reflector underneath the LCD achieves a display device with extremely low power consumption while still exploiting the more advantageous characteristics of liquid crystal. The LCD is thus becoming one of the essential displays used in mobile information terminals.
Moreover, as information volume increases, the demand for color displays for mobile information terminals is increasing. Several proposals on the configuration of reflective LCD for color displays have been made, including the use of color filters and the birefringence effect.
A conventional reflective LCD includes a liquid crystal cell and a pair of polarizer films sandwiching the liquid crystal cell. The light transmissivity of one sheet of the polarizer film is only about 45%, and its transmissivity to light polarized parallel to the absorption axis of the polarizer film is close to 0%. The transmissivity of light polarized perpendicular to the absorption axis is almost 90%. In a reflective LCD using two sheets of polarizer film, light entering the LCD passes through polarizer films four times before exiting the LCD. Accordingly, when non-polarized natural light is incident on the LCD, the overall reflectance or light transmissivity can be calculated as follows, without taking absorption by the color filter into consideration: EQU (0.9).sup.4 .times.50%=32.8%.
To achieve a brighter display, configurations using a single polarizer have been proposed. In such single-polarizer configurations, only one sheet of polarizer film may be disposed on the top side of the liquid crystal cell so that the liquid crystal cell is sandwiched by one sheet of the polarizer film and reflector. For example, this configuration is disclosed in the Japanese Laid-open Patent Nos. H8-201802 and H7-146469. In this case, light entering the reflective LCD passes through the polarizer film only twice. The overall reflectance or light transmissivity of a single-polarizer reflective LCD can therefore be calculated as follows: EQU (0.9).sup.2 .times.50%=40.5%,
again without considering absorption by the color filter.
Compared to the configuration using two sheets of polarizer film, single-polarizer configurations therefore provide up to about 24% improvement ((40.5/32.8).times.100%-100%) in overall reflectance.
Furthermore, the Japanese Laid-open Patent No. H6-308481 proposes a reflective color LCD utilizing the birefringence of twisted liquid crystal layer in combination with polarizer film, to generate a color display without using color filters.
FIG. 5 shows the configuration of a conventional reflective LCD including one sheet of polarizer film (polarizer) and a color filter. A liquid crystal cell 53 is created by sandwiching a liquid crystal layer 57 between a transparent substrate 54, on which a color filter 55 and transparent electrode 56 are formed, and a bottom substrate 59, on which a specular reflector 58 is formed. A retardation film 52, polarizer 51, and forward scattering film 50 are laminated outside this liquid crystal cell to complete the reflective LCD.
If a color filter is used to create a color display in a reflective LCD configuration having two sheets of polarizer film, the reflectance is insufficient to secure the required display brightness. If, to secure the required brightness by increasing the reflectance, the color filter is used in a reflective LCD configuration having a single sheet of polarizer film, creation of an achromatic black and white display may be difficult. Undesired coloring may occur in the conventional configuration. In particular, an achromatic black display with low reflectance may not be achieved. In addition, the optical characteristics of this type of color display depend to a large degree on the direction of incident light and on the viewing angle. If the reflective LCD having a single polarizer film is influenced to a high degree by viewing angle, its disadvantages are not limited to a narrow viewing angle. More specifically, if the reflectance of a black display increases at certain incident light angles, the optical characteristics may be significantly degraded because controlling the incident light angle in a reflective LCD is much more difficult than for a transmissive LCD.
The reflective LCD utilizing the birefringent characteristics of a twisted liquid crystal layer and polarizer films to achieve a color display without a color filter can secure practical brightness even through two sheets of polarizer film are used because no color filter is employed. However, this configuration may not be theoretically applicable to multi-grayscale and multi-color display such as a 16-grayscale, 4096-color display because coloring is produced by the birefringence effect. This type may also have low color purity and limited color reproduction range.
Even in a black and white reflective LCD not employing a color filter, white display with high reflectance may not be achieved if two sheets of polarizer film are used.